Photon noise: observation, squeezing, interpretation

A review of modern concepts of photon noise (PN), which is observed even in the case of an ideal laser, is presented. Methods of transformation, squeezing, and nondemolition observation of PN, are described. The optical nondemolition methods seem to be very important for the interpretation of PN. The experiments with PN suppression by negative electron feedback are analysed in detail within two alternative approaches, which could be called the a priori and a posteriori concepts. According to the first approach, PN exists in the laser beam from the beginning, while according to the second it appears only in the detectors. The theory based on the a priori concept predicts the squeezing of the in-loop field — in contrast to the a posteriori one. Several possible crucial experiments using the nondemolition methods are discussed.